Selecting centrifugal pumps for occaisional psarallel operation?
Selecting centrifugal pumps for occaisional psarallel operation?
(OP)
Hello all, recently, when I was asked a question by a young process engineer, I struggled to answer it concisely. The question is- When selecting 2 x 100% pumps for a given duty, if they may sometimes be run in parallel, we select a pump curve where the single pump case is close to BEP but that the flow/head curve rises to the left hand side of the curve to ensure that at the increased head with both pumps running, both pumps should be running stabily. To what degree should the curve rise? At what slope do we consider the pumps could be possibly unstable when run in parallel operation? I couldn't answer this question properly. Thank you.





RE: Selecting centrifugal pumps for occaisional psarallel operation?
The question is however entirely relavent when discussing 3 x 50% configurations, as well as other configs, 1 x 100% + 2 x 50%, where you obviously could indeed have many desirable occasions to actually run such configurations, so it does warrent some thought, I'm just not sure how much.
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RE: Selecting centrifugal pumps for occaisional psarallel operation?
RE: Selecting centrifugal pumps for occaisional psarallel operation?
if one pump its flow is increased, its pressure drops, so other pumps will increase its flow and flow will settle between the pumps.
consider two pumps with curve with falling pressure and falling flow:
if one pump its flow is increased, its pressure increases,so other pump will decrease flow and decrease pressure.when pump 1 is somewhere at end of curve and its pressure dropped to shut off pressure of 2nd pump,then it all reverses(oscillating).I think API has a standard on prefered curve rise towards shut off pressure,important is to select a pump with rising curve (as opposed to flat or falling curve)towards shut off, when parallel operation is required.
RE: Selecting centrifugal pumps for occaisional psarallel operation?
The algebraic difference of slopes x shutoff head x density of product (units of pressure) should be more than the accuracy of your PT.
That defines it from a controls sensitivity perspective, but there remains some system hysteresis that still isn't resolved. How fast the system curve changes is the result of how fast the head or pressure changes to a unit change of flowrate. That is dependent on where you are on the system curve, the pipe material, length, pipe diameter and product bulk modulus, so that could effectively be anything. There may be no other perspective to examaine this problem from other than from the controls perspective above. Let's see if anyone else has some ideas.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Selecting centrifugal pumps for occaisional psarallel operation?
given that answer to the young operator should make him somewhat reluctant to ask any more difficult questions (lol)
what is PT?
RE: Selecting centrifugal pumps for occaisional psarallel operation?
PT = Pressure Transmitter.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Selecting centrifugal pumps for occaisional psarallel operation?
A simple solution to a simple problem is;
On a graph sheet draw on the pipe system curve to beyond the anticipated flow for the 2 units running in parallel, now draw on the single pump curve and then the 2 pumps in parallel which is the flows added together horizontally at the same head.
Where the single pump and the pumps in parallel cross the system head curve is the operating points for the pumps in single and parallel operation.
RE: Selecting centrifugal pumps for occaisional psarallel operation?
rmw
RE: Selecting centrifugal pumps for occaisional psarallel operation?
RE: Selecting centrifugal pumps for occaisional psarallel operation?
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Selecting centrifugal pumps for occaisional psarallel operation?
RE: Selecting centrifugal pumps for occaisional psarallel operation?
Also he/she could download this free ware and get to see what happens under different scenarios/pump curves
http://ww
RE: Selecting centrifugal pumps for occaisional psarallel operation?